I am using Mongoose since last few months and while querying MongoDB in Node.js and getting results. While doing that I came up with some cases that made me scratch my head for more than couple hours just to figure our it is little ‘gotcha’ (may be because I did not read through all the docs of Mongoose).

Listing my cases below so that if anyone ever comes to same mistake and tries to find for solution online, this can be helpful.

I have tried all following scenarios in 4.0.6 version of Mongoose with MongoDB 3.0.4.

1. When you query using a model and in result you see bunch of fields but when you try to access them you get undefined.

Ex.: (assuming you have user collections with name and email field in it.)

Solution:
You need to add missing field entries in Schema definition, mongoose only binds fields that are in schema definition while creating objects.
Change above schema to following:

var UserSchema = new Schema({
name: String,
email: String
});

And now when you run same example above it should print email id of that particular record.

2. Auto conversion of field values in ‘find’ and ‘aggregate’ query.

Ex.:

var UserSchema = new Schema({
name: String,
cTime: Number
});
var User = mongoose.model('User', UserSchema);
User.find({
cTime: '123456' // here we are passing value as string but it works
// some how mongoose converts it before query may be
}, function(err, users) {
console.log(users); // print users that match the cTime value
});

Now if you try same thing with aggregate function then it will not work:

User.aggregate({
$match: { cTime : '1427807167' } // mongoose does not convert it to number this time
}, function(err, users) {
console.log(users); // this will be empty, no match
});

In case of aggregate you have to take care of conversion by your own. Most likely case for this is getting time or other numeric values in req.query and then directly using it in $match query of aggregate. Instead you should first convert it to specific type of field and then use that in query.

User.aggregate({
$match: { cTime : parseInt(req.query.time) } // assuming req.query.time has the value for cTime
}, function(err, users) {
console.log(users); // now this will be not empty and prints the matching records.
});

3. Updating array items and calling save

When you just update an item in array of objects and call save on it, it will update the array in database. To make it work you need to mark the array field as modified.
user.markModified(‘array’); // this will mark array field of user object
if you call user.save after this then it will update array in database.

4. Listen to db connection drop event

This is probably not a gotcha, but it really made me work about an hour before knowing the solution. When connection to database is dropped for some reason then you will get it as ‘error’ event and not in ‘disconnected’ event. I tried to listed to ‘disconnected’ event and do reconnect but found that it does not even call it when db connection is crashed for some reason. It listens on ‘error’ event in that case so if you are planning to catch event for db connection failure and do operation based on it.

That’s all for now.

If you have seen any more while working with it, do let all know by commenting on the post. I will also update it with more as I come to know.